A standard box cutter as described in German patent 3,622,342 has a housing in which a blade secured in a blade holder or slide can move longitudinally. A laterally projecting pusher button projects from the slide and is actuated by the user's thumb to extend the blade from the housing. A tension spring connected between the blade slide and the housing continuously urges the blade back into a retracted position in the housing.
Thus to use this knife the user pushes forward the blade slide with his or her thumb to extend the blade and engages the blade in the item being cut. Normally the friction of the blade against the workpiece is sufficient to overcome the spring force so that, once the blade is actually cutting, the user can release the blade holder and the blade will remain in the extended position. Once the blade is pulled out of the workpiece it will snap back into the retracted position so the knife can be pocketed without danger.
The problem with such a system is that the user frequently maintains his or her thumb on the button as the knife is used, normally because the knife is used immediately on the workpiece when the blade is extended and the user has no reason to change his or her grip. Thus if the button is not released, a subsequent attempt to pocket the knife, for instance, can result in injury.